1. Field of the Invention
The present invention relates to a sheet supply device.
2. Description of the Related Art
A sheet supply device is used in image forming devices and other office equipment. As shown in FIGS. 1(a) and 1(b), a conventional sheet supply device includes a sheet supply roller 80, an idle roller 82, and a rotational shaft 84, that all rotate around the same axis. The sheet supply roller 80 has a large radius portion 80a and a small radius portion 80b. The large radius portion 80a has a radius greater than the radius of the idle roller 82. The small radius portion 80b has a radius smaller than the radius of the idle roller 82. When the rotational shaft 84 is driven to rotate, the sheet supply roller 80 rotates along with the rotational shaft 84. On the other hand, the idle roller 82 is freely rotatable around the rotational shaft 84. A stack of sheets S are provided at a predetermined sheet supply position beneath the idle roller 82. Although not shown in the drawings, an urging member is disposed at a side of the sheet S opposite from the idle roller 82, and urges the sheet S toward the idle roller 82.
When sheet supply operations are not being performed, the small radius portion 80b of the sheet supply roller 80 confronts the surface of the sheet S at the sheet supply position, and the idle roller 82 contacts the surface of the sheet S. In this way, the idle roller 82 maintains a minimum distance between the rotational shaft 84 and the sheet S.
When sheet supply operations are started, the rotational shaft 84 is driven to rotate in a direction indicated by an arrow B in FIG. 1(a), and the sheet supply roller 80 rotates in association with the rotational shaft 84. As a result, as shown in FIG. 1(a) the large radius portion 80a of the sheet supply roller 80 comes into contact with the sheet S, and the idle roller 82 is separated from the sheet S. As the sheet supply roller 80 further rotates, the sheet S is fed toward the sheet feed rollers 86, 88 because of friction between the sheet supply roller 80 and the sheet S. When the front edge of the sheet S reaches the sheet feed rollers 86, 88, the sheet feed rollers 86, 88 pick up the sheet S and further transport the sheet S.
After the sheet S is picked up by the sheet feed rollers 86, 88, the small radius portion 80b again comes into confrontation with the sheet S. As a result, as shown in FIG, 1(b), the idle roller 82 comes into contact with the sheet S, and the sheet supply roller 80 is separated from the sheet S. Then, the rotation of the sheet supply roller 80 is stopped. However, because the idle roller 82 is freely rotatable about the rotational shaft 84, the idle roller 82 is rotated as the sheet S is fed by the sheet feed roller 86, 88 because of friction generated between the idle roller 82 and the sheet S. Accordingly, the sheet S is smoothly transported without any undesirably large force being generated between the idle roller 82 and the sheet S.
Usually, the large radius portion 80a of the sheet supply roller 80 is formed from a soft material, such as rubber, that has a large friction coefficient. Therefore, when the sheet supply roller 80 is used for a long period of time, the outer peripheral surface of the large radius portion 80a is gradually worn down by friction. On the other hand, the idle roller 82 is normally formed from a material with a low friction coefficient, and that is much harder than the material of the sheet supply roller 80. For this reason, the idle roller 82 wears down much slower than the sheet supply roller 80. Accordingly, the difference between the radius of the large radius portion 80a and the radius of the idle roller 82 is gradually reduced with passage of time.
The relationship of the radius of the large radius portion 80a and the radius of the idle roller 82 with respect to the passage of time is shown in graphical form in FIG. 2. After the sheet supply roller 80 has been operated for a cumulative operation time of t1, the difference between the radius of the large radius portion 80a and the idle roller 82 reaches a minimum tolerance value A. When the difference is reduced to lower than the minimum tolerance value A, sheets S will quite frequently be improperly picked up by the sheet supply roller 80, thereby preventing proper sheet supply operations.
Therefore, the sheet supply roller 80 must be replaced with a new one before the cumulative operation time of the sheet supply roller 80 exceeds the value t1. However, the sheet supply roller 80 is expensive to replace, so it is desirable to increase the life of the sheet supply roller 80.